1. Field of the Invention
The present invention relates to a method for measuring in situ the residual tension F.sub.1 of a pre-stressed bar braced between two anchorages, by determining the two-way propagation time T.sub.1 of an ultra-sonic wave between the two longitudinal ends of the bar and by evaluating the residual tension F.sub.1 by a formula of the form F.sub.1 =F.sub.0 +k.sub.b. (T.sub.1 -T.sub.0) where F.sub.0, T.sub.0 and k.sub.b are constants determined during a preliminary calibration carried out without demounting the bar.
2. Description of Related Art
A process of this type is described in the French patent 2 750 498. According to this patent, the preliminary calibration consists in:
applying to one end of the pre-stressed bar a traction effort F which progressively increases; PA1 measuring simultaneously the longitudinal displacement A of the end of the pre-stressed bar as well as the above-mentioned traction effort whereby these measurements correspond to a curve of the traction effort F as a function of the displacement A which presents an initial portion substantially following a first straight line having a high slope, then a curved portion and finally a final portion substantially following a second straight line having a lower slope; PA1 determining the residual tension as being the value of the traction effort F which corresponds to the intersection of the two straight lines; PA1 applying successively to the pre-stressed bar at least two known and distinct tension values higher than the residual tension, by traction on the end of said pre-stressed bar; PA1 measuring for each of these tension values the two-way propagation time of an ultra-sonic wave between the two longitudinal ends of the pre-stressed bar; PA1 and deducing from these measurements the constants of the evaluation formula of the residual tension. PA1 subjecting the bar to different values of a traction effort F, applied in a zone situated outside of the interval between the two anchorages; PA1 for each value of the applied traction effort F, measuring the two-way propagation time T of an ultra-sonic wave between the two longitudinal ends of the bar, in such a way as to memorize points of a curve associating the traction effort F with the two-way propagation time T; PA1 determining the constant T.sub.0 as being the two-way propagation time T measured for a traction effort F=0; PA1 approaching said curve, in the neighborhood of the traction effort value F=0, by a first straight line having a slope k.sub.a ; PA1 approaching an upper portion of said curve by a second straight line of inclination k.sub.L ; PA1 determining the constant F.sub.0 as being the traction effort F corresponding to the point of intersection of said first and second straight lines; and PA1 determining the constant k.sub.b according to the equation 1/k.sub.b =(1/k.sub.L)-(1/k.sub.a).
Owing to these measures, it is necessary to apply traction efforts to the end of the pre-stressed bar only during calibration, which is carried out once for all.
Once the calibration has been carried out, one can subsequently check as often as necessary the residual tension of the pre-stressed bar solely by measuring the two-way propagation time of an ultra-sonic wave between the two ends of this bar. These subsequent checks are therefore very simple and very quick. This process is well adapted to cases where there are restrictions to the access to the structure, for example due to ionising rays.
An object of the present invention is to improve the accuracy of the processes for measuring residual tension by ultra-sound.